SHA password hashes

The Secure Hash Algorithms (SHA) are a set of hash functions often used to hash passwords. By default Arch uses SHA-512 for passwords, but some systems may still be using the older MD5 algorithm. This article describes how to increase password security.

Benefits of SHA-2 over MD5

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In Linux distributions login passwords are commonly hashed and stored in the /etc/shadow file using the MD5 algorithm. The security of the MD5 hash function has been severely compromised by collision vulnerabilities. This does not mean MD5 is insecure for password hashing but in the interest of decreasing vulnerabilities a more secure and robust algorithm that has no known weaknesses (e.g. SHA-512) is recommended.

The following tutorial uses the SHA-512 hash function, which has been recommended by the United States' National Security Agency (NSA) for Red Hat Enterprise Linux 5. Alternatively, SHA-2 consists of four additional hash functions with digests that are 224, 256, 384, and 512 bits.

Increasing security

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Note: With shadow 4.1.4.3-3 sha512 is the default for new passwords (see bug 13591).

If your current password was created with shadow version prior to 4.1.4.3-3 (2011-11-26) you are using MD5. To start using a SHA-512 hash you just need to change your password with passwd.

Note: You must have root privileges to edit this file.

The rounds=N option helps to improve key strengthening. The number of rounds has a larger impact on security than the selection of a hash function. For example, rounds=65536 means that an attacker has to compute 65536 hashes for each password he tests against the hash in your /etc/shadow. Therefore the attacker will be delayed by a factor of 65536. This also means that your computer must compute 65536 hashes every time you log in, but even on slow computers that takes less than 1 second. If you do not use the rounds option, then glibc will default to 5000 rounds for SHA-512. Additionally, the default value for the rounds option can be found in sha512-crypt.c.

Open /etc/pam.d/passwd with a text editor and add the rounds option at the end of of the uncommented line. After applying this change the line should look like this:

password required pam_unix.so sha512 shadow nullok rounds=65536

Note: For a more detailed explanation of the /etc/pam.d/passwd password options check the pam_unix(8) man page.

Re-hash the passwords

Even though you have changed the encryption settings, your passwords are not automatically re-hashed. To fix this, you must reset all user passwords so that they can be re-hashed.

As root issue the following command,

# passwd username

where username is the name of the user whose password you are changing. Then re-enter their current password, and it will be re-hashed using the SHA-2 function.

To verify that your passwords have been re-hashed, check the /etc/shadow file as root. Passwords hashed with SHA-256 should begin with a $5 and passwords hashed with SHA-512 will begin with $6.